The present invention is directed toward the apparatus and the method of separating fine particles which are capable of moving in an electrostatic field. The particles as they move through the electro-static field are caused to oscillate in response to the forces produced by the field. The oscillating particles are then separated or classified by passing them through classifying screens which are charged.
The concept of passing particles through an electrostatic field for the purpose of propelling the particles beyond a screen is disclosed in U.S. Pat. No. 3,635,340 to Dunn. This patent discloses the use of particle momentum produced by pulling the particles across a field to propel particles through a printing screen. Further, it is also disclosed that this propulsion of the particles beyond a second electrode may be used for possible particle classification. This patent, however, does not recognize or utilize particle oscillation as the vehicle for screen trials. It was further disclosed that particle separation could be accomplished by passing the particles across a horizontal conveying electrode which relied upon vibration to move the particles to the screen or stencil electrode mounted above the horizontal vibrating electrode.
Another example of the use of electrostatic separation of particles known in the prior art is shown in U.S. Pat. No. 2,361,946 to Johnson et al. The Johnson et al patent discloses an electrostatic separation of particles which utilizes direct fields or alternating fields for the production of particle dispersion, agitation and propulsion between electrodes. The Johnson et al patent utilizes an inclined electrode configuration where the sieve electrode is placed below an upper electrode. Where it is desired to use direct potentials, the upper electrode is a bare, solid, metallic electrode. A solid upper electrode has been found to be required in apparatus which use an inclined electrode configuration. The use of such electrodes, however, allows fine particles to adhere to the surface in local areas and thus produces variations in the electrical field strength, and sparking and possible stoppage of the process.
In the Johnson et al patent, the principle phenomena relied upon is the attraction and repulsion of particles between electrodes of an opposite charge. A particle by reason of the charge received from the lower sieving electrode is propelled upwardly to the upper electrode plate from which, by contact therewith, it receives the opposite charge and is propelled back down to the lower electrode. Particles which do not actually touch the upper electrode may also be propelled downwardly by gravity.
In the Johnson et al patent, there is no recognition of the potential use of the inherent oscillation of a dispersed group of particles between electrodes of a like charge.
The prior art has also utilized electrostatic fields for the separation of particles through the technique of passing the particle through a field and relying upon the mass-to-charge ratio to accomplish the separation. An example of this is found in U.S. Pat. No. 2,803,344 to Morrison, which utilizes gravity to separate the particles as they pass across an electrostatic field. This technique, however, does not rely upon the oscillating motion produced by the electrostatic dispersion to propel the particles to a classifying screen. In this apparatus, there is no requirement that the particles oscillate during separation since there is no classification screen against which trails are made.